1. Field of the Invention
This invention relates to an optical device for electrostatic copying machines with a copy enlarging and reducing function, and more particularly to an optical device for a copying machine in which the position of the principal point of the optical system can be inverted for copy enlargements and reductions.
2. Description of the Prior Art
Recently, electrostatic copying machines with the functions of enlarging and reducing the copy ratio form a main stream in the art. In a copying machine with such copy enlarging and reducing functions, the optical system which is interposed between an original mount and a sensitive material surface for exposure, e.g. a photosensitive drum surface, usually employs a symmetrical lens construction which has an principal point internally of the optical system like the so-called Orthometa- or Sellar-type lens. The optical system which is movable along the light path is located at a median position between the original mount and the sensitive material surface at the time of duplication in 1:1 copy ratio, shifting the optical system toward the original mount and the sensitive material surface at the time of copy enlargements and reductions, respectively, thereby varying the ratio of the light path length on the side of the object point or between the optical system and the original mount to the light path length on the side of the image point or between the optical system and the sensitive material surface according to changes in copy ratio.
In order to make the copying machine compact, more than one mirror is provided in each of the light paths on the sides of the object and image points, thereby folding the light paths in such a manner as to reduce the space for the entire light path length. In case of a copying machine of the type which has a fixed original mount and a movable light source for illuminating the original, some of the mirrors are moved to follow the movement of the light source.
Therefore, when shifting the positions of the optical system and mirrors to alter the copy ratio as mentioned hereinbefore, it is necessary to make arrangements to avoid interferences therebetween. That is to say, restrictions based on a selected range of copy ratios are imposed on the positional relationship between the optical system and the mirrors which are located forward and backward of the optical system. In this connection, although currently there is an extremely strong demand for small-sized and compact copying machines, the reduction of the machine size has a limit in view of the restrictions on the positional relationship of the optical system with the forward and backward located mirrors, which has a great influence on the dimensions of the copying machine. Otherwise, there is another problem that the designed range of the copy ratios has to be sacrificed to some extent for reduction of the machine size.